使用 Paho C 连接到部署

本文主要介绍如何在 C 项目中使用 Eclipse Paho C ，实现客户端与 MQTT 服务器的连接、订阅、收发消息等功能。

Eclipse Paho C 与 Eclipse Paho Embedded C 均为 Eclipse Paho 项目下的 C 语言客户端库（MQTT C Client），均为使用 ANSI C 编写的功能齐全的 MQTT 客户端。

前置准备

MQTT 服务器部署

使用 EMQX 提供的免费公共 MQTT 服务器，该服务基于 EMQX 的全托管 MQTT 云服务创建。服务器接入信息如下：
- 连接地址: broker.emqx.io
- TCP Port: 1883
- SSL/TLS Port: 8883
您也可以自己创建部署，在部署概览下可以查看到连接相关的信息，请确保部署状态为运行中。使用 TCP 端口或 SSL/TLS 端口测试连接到 MQTT 服务器。如果您是自己创建部署，请设置认证，在部署控制台认证鉴权 > 认证中设置用户名和密码，用于连接验证。

安装依赖

bash

sudo apt-get update
sudo apt-get -y install build-essential git cmake

编译 Paho MQTT 库

Linux/Mac

bash

git clone https://github.com/eclipse/paho.mqtt.c.git
cd org.eclipse.paho.mqtt.c.git
make
sudo make install

Windows

mkdir build.paho

cd build.paho

call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x64

cmake -G "NMake Makefiles" -DPAHO_WITH_SSL=TRUE -DPAHO_BUILD_DOCUMENTATION=FALSE -DPAHO_BUILD_SAMPLES=TRUE -DCMAKE_BUILD_TYPE=Release -DCMAKE_VERBOSE_MAKEFILE=TRUE ..

nmake

通过 TCP 端口连接

本节介绍了如何通过 TCP 端口连接到部署。

导入所需库函数

#include "stdlib.h"
#include "string.h"
#include "unistd.h"
#include "MQTTClient.h"

定义连接地址、认证信息以及消息发布和接收主题

// Enable or disable SSL/TLS connection (1 for SSL/TLS, 0 for TCP)
#define USE_SSL 0

#if USE_SSL
#define ADDRESS "ssl://broker.emqx.io:8883"
#else
#define ADDRESS "tcp://broker.emqx.io:1883"
#endif

#define USERNAME    "emqx"
#define PASSWORD    "public"
#define CLIENTID    "c-client"
#define QOS         0
#define TOPIC       "emqx/c-test"
#define TIMEOUT     10000L

开始连接 / 编写 MQTT 代码

int rc;
MQTTClient client;

MQTTClient_create(&client, ADDRESS, CLIENTID, 0, NULL);
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
conn_opts.username = USERNAME;
conn_opts.password = PASSWORD;

if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
    printf("Failed to connect, return code %d\n", rc);
    exit(-1);
} else {
    printf("Connected to MQTT Broker!\n");
}

通过 SSL/TLS 协议连接

本节介绍了如何通过 SSL/TLS 端口连接到部署。

调整连接地址，通过将 USE_SSL 设置为1实现。

// Enable or disable SSL/TLS connection (1 for SSL/TLS, 0 for TCP)
#define USE_SSL 1

开始连接 / 编写 MQTT 代码，

通过函数 configureSSLOptions() 来设置SSL选项。如果需要配置服务器证书，可以通过 ssl_opts.trustStore 加载证书。如果不需要，将 ssl_opts.enableServerCertAuth 设置为0。

MQTTClient_SSLOptions configureSSLOptions() {
    MQTTClient_SSLOptions ssl_opts = MQTTClient_SSLOptions_initializer;
    ssl_opts.enableServerCertAuth = 1;
    // CA_CERTIFICATE_FILE_Path specify server CA
    // ssl_opts.trustStore = CA_CERTIFICATE_FILE_Path;
    return ssl_opts;
}

···

#if USE_SSL
    MQTTClient_SSLOptions ssl_opts = configureSSLOptions();
    conn_opts.ssl = &ssl_opts;
#endif

订阅和发布

本节主要介绍了如何在已连接到部署的情况下订阅主题并发布消息。

订阅主题

设置将要订阅的主题及对应 QoS 等级。

// subscribe topic
MQTTClient_subscribe(client, TOPIC, QOS);

取消订阅

通过以下代码取消订阅，此时应指定取消订阅的主题。

MQTTClient_unsubscribe(client, TOPIC);

发布消息

发布消息时需要告知 MQTT Broker 目标主题及消息内容。

void publish(MQTTClient client, char *topic, char *payload) {
    MQTTClient_message message = MQTTClient_message_initializer;
    message.payload = payload;
    message.payloadlen = strlen(payload);
    message.qos = QOS;
    message.retained = 0;
    MQTTClient_deliveryToken token;
    MQTTClient_publishMessage(client, topic, &message, &token);
    MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Send `%s` to topic `%s` \n", payload, TOPIC);
}

// Call the `publish` function in a loop to finish publishing the message
char payload[16];
for (int i = 0; i < 100; i += 1) {
    // publish message to broker
    snprintf(payload, 16, "message-%d", i);
    publish(client, TOPIC, payload);
    sleep(1);
}

接收消息

通过以下代码指定客户端对消息事件进行监听，并在收到消息后执行回调函数，将接收到的消息及其主题打印到控制台。

MQTTClient_setCallbacks(client, NULL, NULL, on_message, NULL);
···
// Define the on_message callback function to print the content of the messages received by the subscribed topic
int on_message(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    char *payload = message->payload;
    printf("Received `%s` from `%s` topic \n", payload, topicName);
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}

断开连接

如客户端希望主动断开连接，可以通过如下代码实现：

MQTTClient_disconnect(client, TIMEOUT);
MQTTClient_destroy(&client);

以上部分仅列出了一些关键代码，项目完整代码请见：[mqtt-client-C-paho]https://github.com/emqx/MQTT-Client-Examples/tree/master/mqtt-client-C-paho)，您可以进行下载并体验。

完整代码

#include "stdlib.h"
#include "string.h"
#include "unistd.h"
#include "MQTTClient.h"

// Enable or disable SSL/TLS connection (1 for SSL/TLS, 0 for TCP)
#define USE_SSL 1

#if USE_SSL
#define ADDRESS "ssl://broker.emqx.io:8883"
#else
#define ADDRESS "tcp://broker.emqx.io:1883"
#endif

#define USERNAME    "emqx"
#define PASSWORD    "public"
#define CLIENTID    "c-client"
#define QOS         0
#define TOPIC       "emqx/c-test"
#define TIMEOUT     10000L

MQTTClient_SSLOptions configureSSLOptions() {
    MQTTClient_SSLOptions ssl_opts = MQTTClient_SSLOptions_initializer;
    ssl_opts.enableServerCertAuth = 1;
    ssl_opts.trustStore = CA_CERTIFICATE_FILE;
    return ssl_opts;
}


void publish(MQTTClient client, char *topic, char *payload) {
    MQTTClient_message message = MQTTClient_message_initializer;
    message.payload = payload;
    message.payloadlen = strlen(payload);
    message.qos = QOS;
    message.retained = 0;
    MQTTClient_deliveryToken token;
    MQTTClient_publishMessage(client, topic, &message, &token);
    MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Send `%s` to topic `%s` \n", payload, TOPIC);
}

int on_message(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    char *payload = message->payload;
    printf("Received `%s` from `%s` topic \n", payload, topicName);
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}

int main(int argc, char *argv[]) {
    int rc;
    MQTTClient client;

    MQTTClient_create(&client, ADDRESS, CLIENTID, 0, NULL);
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    conn_opts.username = USERNAME;
    conn_opts.password = PASSWORD;

#if USE_SSL
    MQTTClient_SSLOptions ssl_opts = configureSSLOptions();
    conn_opts.ssl = &ssl_opts;
#endif

    MQTTClient_setCallbacks(client, NULL, NULL, on_message, NULL);
    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
        exit(-1);
    } else {
        printf("Connected to MQTT Broker!\n");
    }
    // subscribe topic
    MQTTClient_subscribe(client, TOPIC, QOS);
    
    char payload[16];
    for (int i = 0; i < 10; i += 1) {
        // publish message to broker
        snprintf(payload, 16, "message-%d", i);
        publish(client, TOPIC, payload);
        sleep(1);
    }
    MQTTClient_unsubscribe(client, TOPIC);
    MQTTClient_disconnect(client, TIMEOUT);
    MQTTClient_destroy(&client);
    return rc;
}

测试验证

编写 CMakeLists.txt

cmake_minimum_required(VERSION 3.17)
find_package(eclipse-paho-mqtt-c 1.3.9 REQUIRED)
project(mqtt_c C)
include_directories(/usr/local/include)
link_directories(/usr/local/lib)
set(CMAKE_C_STANDARD 99)
add_executable(mqtt_c main.c)
target_link_libraries(mqtt_c paho-mqtt3c)

编译和运行代码

创建专有版部署

创建旗舰版部署

扩展认证

扩展授权

单点登录 (SSO)

告警

CoAP 协议网关

平台 API

部署 API

Serverless 部署 API

使用 Paho C 连接到部署

前置准备

MQTT 服务器部署

安装依赖

编译 Paho MQTT 库

Linux/Mac

Windows

通过 TCP 端口连接

通过 SSL/TLS 协议连接

订阅和发布

订阅主题

取消订阅

发布消息

接收消息

断开连接

完整代码

测试验证

更多内容

Serverless 部署 API

使用 Paho C 连接到部署 ​

前置准备 ​

MQTT 服务器部署 ​

安装依赖 ​

编译 Paho MQTT 库 ​

Linux/Mac ​

Windows ​

通过 TCP 端口连接 ​

通过 SSL/TLS 协议连接 ​

订阅和发布 ​

订阅主题 ​

取消订阅 ​

发布消息 ​

接收消息 ​

断开连接 ​

完整代码 ​

测试验证 ​

更多内容 ​

使用 Paho C 连接到部署

前置准备

MQTT 服务器部署

安装依赖

编译 Paho MQTT 库

Linux/Mac

Windows

通过 TCP 端口连接

通过 SSL/TLS 协议连接

订阅和发布

订阅主题

取消订阅

发布消息

接收消息

断开连接

完整代码

测试验证

更多内容